Councillor's gutted by Oxford's river disaster (6)
I believe the answer is:
crisis
'disaster' is the definition.
(both can mean a terrible event)
'councillor's gutted by oxford's river' is the wordplay.
'gutted' suggests removing the centre (to gut something is to remove its insides).
'by' means one lot of letters go next to another.
'oxford's river' becomes 'isis' (term for the Thames used in Oxford).
'councillor' with its middle taken out is 'cr'.
'cr'+'isis'='CRISIS'
(Other definitions for crisis that I've seen before include "unstable period" , "Moment of danger" , "Crucial or decisive moment" , "Time of intense difficulty or danger" , "Climax, breaking point" .)